1. Field of the Invention
Non-contacting position sensing.
2. Description of Related Art
Most advances in modern technology result from being able to make accurate measurements. State-of-the-art technology is capable of producing highly precise products only because it is capable of measuring dimensions and positions with great accuracy. The integrated circuit industry depends on making and superimposing submicron patterns with great precision. Computer controlled milling machines produce complicated and precisely shaped workpieces. Metrology, the technology of measurement, is the quality control that makes modern technology possible.
Some measurement tools require direct contact. Examples are calipers, strain gauges, atomic force microscopes, contact profilometers. Others operate without contact. Examples are capacitive gauges, microscopes, optical interferometers and telescopes.
In general a remote instrument cannot make as accurate a measurement as one close to an object. An exception is the interferometer, which can measure changes in position parallel to the interferometer beam to a fraction of a wavelength of light. However, interferometric measurement is suitable only for a special category of objects--those with optically perfect planar surfaces.
One approach to remote measurement compares markings--one on the target and another on a reference body. An interference grating is a useful marking. Its inherent redundancy makes for ready identification and reliable sensing. It has been recognized that grating comparison may be self registering. SPIE, Vol. 1088, pps. 258-267 (1989) describes a complex system entailing reflection and route retracing which compares a conventional grating with a segmented grating--a two-part grating with parts out of phase. Positioning is based on relative degree of overlap for the two segments. However, route retracing again requires perfect reflecting surfaces which are optically flat and clean.
There is a continuing need for a robust method for remote position sensing.